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Full text of "Text Book Of Mechanical Engineering"

288

Crystallisation produced by Punching.

being drawn with sides as i : 6.    Then if d be diameter of punch,
and / plate thickness, dą will be the size of hole in bolster, or

ii

The material removed from the plate is known as the
c punching,' or ' burr/ and during the operation a certain portion
is compressed into the surrounding plate, thereby increasing its
density and causing i distress ; ' the clearance between punch and
bolster hole is to prevent this, which it does partially. The dis-
tressed area is said to be small, and the distressment relievable by
rimering, annealing, or both. Dr. Kirk's experiments in 1877 on
the fracture of punched plates, shewed the crystalline or weak
portion varying between the two limits at F, Fig. 282. All this
was removed by subsequent annealing, heating to redness, and
slowly cooling.

But the question was raised : if the plates require such treat-
ment after punching, and alignment be not then obtainable unless
punched after rolling (very difficult with machines as made), why
not drill them at once and avoid annealing ? There is no difficulty
in drilling after bending, and further, the holes may be made
through both thicknesses of plate at once, thus securing accuracy
of position. Drilling * in position ' is therefore the present-
day practice, and we are not aware of any workshop where
punching is performed except for very thin plates, or for
roughing out man-holes, &c. After drilling, the sharp edge is
taken off by a countersinking tool, or rosebit, to prevent cutting
action on the rivet, caused by expansion and contraction of the
boiler.

Shearing causes the same harm to the plate as punching,
and the edges should always be planed afterwards.

D Cramps as at A, Fig. 219^, are required by boiler
makers for temporarily fastening plates together, or for providing
a hold when slinging.

Machine Tools, as explained in Chapter V., are daily
gaining ground, to increase the output, while securing greater
accuracy and cheaper production. As in the- Fitting shop, they
were at first driven entirely by belts from a line of shafting, but
the intermittent demand renders hydraulic power more advan-